1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, specifically to a high voltage MOS (Metal Oxide Semiconductor) transistor with an increased driving capacity for a level shifter used in an LCD (Liquid Crystal Display) driver, an EL (Electro Luminescence) driver and the like.
2. Description of the Related Art
A semiconductor device according to a conventional art will be explained hereinafter referring to a cross-sectional view of an LOCOS (Local Oxidation of Silicon) offset type high voltage MOS transistor shown in FIG. 9.
A gate electrode 53 is formed on a first gate insulation film 52A and a second gate insulation film 52B on a semiconductor layer 51 of a first conductivity (P-type in this example), as shown in FIG. 9. An N+ source region 54 is formed adjacent one edge of the gate electrode 53. An Nxe2x88x92 drain region 56 is formed facing to the N+ source region 54 across a channel region 55. And an N+ drain region 57 is formed within the Nxe2x88x92 drain region 56 and apart from the other edge of the gate electrode 53. A numeral 58 refers to a device isolation film.
The second gate insulation film 52B is thicker than the gate insulating film of a normal voltage (e.g. 10V) MOS transistor. That is to say, for instance, the gate insulation film of the high voltage MOS transistor is about 120 nm thick while that of the normal voltage MOS transistor is about 15 nm thick.
A LOCOS insulation film (the second gate insulation film 52B) is formed on the Nxe2x88x92 drain region in order to increase a maximum voltage applicable to the device by relieving an electric field concentration in that area.
A weak inversion leakage current increases when the gate insulation film becomes thicker. Therefore raising a threshold voltage by forming an ion-implanted layer in the channel region is necessary in order to suppress the leakage current.
However, on the other hand, when the threshold voltage is raised, a driving capacity of the transistor is reduced.
This invention is directed to a semiconductor device with improved driving capacity and its manufacturing method.
The semiconductor device of this invention has a gate electrode formed on a gate insulation film on a semiconductor layer, and source and drain regions formed adjacent the gate electrode, while the gate insulation film has three or more different thicknesses.
The semiconductor device of this invention has a first gate insulation film formed by a selective oxidation and second gate insulation film formed by a thermal oxidation on the semiconductor layer. The gate electrode is formed across the first and the second gate insulation film, wherein the second gate insulation film consists of a plurality of gate insulation films of different thicknesses.
The second gate insulation film consists of a thicker second gate insulation film and a thinner second gate insulation film, and the first gate insulation film is thicker than the thicker second gate insulation film.
In a manufacturing process of the semiconductor device of this invention, which has the first gate insulation film formed by a selective oxidation and the second gate insulation film formed by a thermal oxidation of the semiconductor layer and the gate electrode formed across the first and the second gate insulation films, the second gate insulation film is formed in the following steps. The thicker second gate insulation film is formed on the semiconductor layer. A predetermined area of the thicker second gate insulation film is removed. Then, the thinner second gate insulation film is formed bordering on the thicker second gate insulation film.
In a manufacturing method of the semiconductor device of this invention, the second gate insulation film can be formed after the first gate insulation film is formed.
In a manufacturing method of the semiconductor device of this invention, the second gate insulation film can be formed before the first gate insulation film is formed.
A manufacturing method of the semiconductor device of this invention has a process to form a LOCOS insulation film by selective oxidation of a semiconductor layer with a mask of an oxidation resistant film formed on the semiconductor layer, a process to form a thicker gate insulation film bordering on the LOCOS insulation film after removing the oxidation resistant film, a process to form a thinner gate insulation film bordering on the thicker gate insulation film by thermal oxidation after removing a predetermined portion of the thicker gate insulation film, a process to form a gate electrode across the thinner gate insulation film, thicker gate insulation film and the LOCOS insulation film and a process to form source and drain regions adjacent the gate electrode.
A LOCOS insulation film is formed by selective oxidation of the semiconductor layer using an oxidation resistant film as a mask, after forming an insulation film or an insulation film and a polysilicon film on the semiconductor layer.